In recent years, there has been a demand for finer display devices for use in AV devices and OA devices, and making the screen of these devices larger. As such display devices, a variety of display devices such as a display device adopting the Cathode Ray Tube (CRT) system, a liquid crystal display device (LCD), a plasma display device (PDP), an electroluminescense (EL) display device, and a light-emitting diode (LED) display device are available. In these display devices, in response to making the screen larger, an increase in weight, dimensions, and power consumption are expected, and so there has been a demand for, in addition to enlarging the screen, making the display devices lighter, thinner, and less power consuming.
Of these display devices, the liquid crystal display device (referred to as LCD hereinafter) has such advantages in that, compared with other display devices, the thickness (depth) is far thinner, the power consumption is low, and a full color can be realized with ease. For this reason, the LCD is suitable for display devices having a larger screen such as a large monitor and a wall display device, and is considered to be the best candidate for realizing a larger screen.
However, in the LCD, when the screen is made larger, the fraction defective is abruptly increased due to breakage of a signal wire and a pixel failure in the manufacturing process. Also, the manufacturing process is complicated as a result of screen enlargement. This presents a problem that the price of the LCD is increased.
In order to solve this problem, an LCD in which a larger screen is realized by connecting a plurality of small substrates with each other has been suggested. The LCD in which a larger screen is realized in this manner has an arrangement wherein at least one of a pair of substrates having electrodes constituting the LCD is a large connected substrate which is prepared by connecting a plurality of small substrates on the sides.
Particularly, in the LCD of active-matrix type, it is extremely difficult to manufacture an active-matrix substrate, which is a substrate provided with fine active elements per pixel, having a large area with a good yield. In order to overcome this difficulty, the active-matrix substrate is manufactured by connecting a plurality of small substrates with each other on the sides so as to realize a single connected substrate (large active-matrix substrate). In this manner, by providing the active-matrix substrate, which is difficult to manufacture in a large size, in the form of a connected substrate, it is possible to improve the efficiency in productivity of the LCD having a large screen.
As such LCD adopting the connected substrate, a liquid crystal panel disclosed in Japanese Examined Utility Model No. 28086/1992 (Jitsukouhei 4-28086), and an LCD and manufacturing method thereof disclosed in Japanese Unexamined Patent publication No. 184849/1996 (Tokukaihei 8-184849) are available.
For example, the LCD disclosed in Japanese Unexamined Patent publication No. 184849/1996 (Tokukaihei 8-184849) has an arrangement wherein, as shown in FIG. 19(a) and FIG. 19(b), a single large substrate 61, composed of four small substrates 61a connected to each other in a "tile" manner, is combined with a counter substrate 62 via a liquid crystal layer 64 which is sealed by a seal material 63. In this LCD, a supporting member 66 is provided between a connected portion of the small substrates 61a and a non-transmissive pattern 65 formed on the counter substrate 62.
To explain more specifically, as shown in FIG. 19(b), a plurality of small substrates 61a (four in FIG. 19(a) and FIG. 19(b)) brought into close proximity with each other on a single plane are positioned so as to face the single counter substrate 62. On the counter substrate 62, there is provided the non-transmissive pattern 65 in the form of a grid corresponding to a region surrounding each pixel. The facing side edges of the plurality of small substrates 61a are bonded with one another by a fixing adhesive 67. Also, the seal material 63 is provided on the periphery on the surface of the counter substrate 62 facing the small substrates 61a, and the liquid crystal layer 64 is sealed by the seal material 63 in a spacing between the counter substrate 62 and the small substrates 61a bonded with each other. On respective outer surfaces of the small substrates 61a bonded with each other and the counter substrate 62, a polarizing plate 68 is provided. Also, the supporting member 66 is provided between the connected portion (seam) by the adhesive 67 and the non-transmissive pattern 65 facing the connected portion.
Note that, as the supporting member 66 provided on the sides of the small substrates 61a connected to each other, a material in which a space keeper (spacer) (not shown) is added to the same resin as that of the seal material 63 is adopted. Also, as the resin of the seal material 63 and the supporting member 66, UV curable resin or heat curable resin is adopted. The seal material 63 and the supporting member 66 are generally formed in a predetermined pattern using a screen printing method or dispense profiling method.
The following describes manufacturing steps of the LCD. First, the seal material 63 and the non-transmissive pattern 65 are formed on the counter substrate 62. Thereafter, the small substrates 61a are connected to each other by the adhesive 67. Then, on the non-transmissive pattern 65 formed on a position corresponding to the connected portion of the small substrates 61a, the supporting member 66 made of the same material as that of the seal material 63 is formed.
Therefore, the supporting member 66 is provided between (a) the connected portion of the small substrates 61a connected to each other by the adhesive 67 and (b) the non-transmissive pattern 65 facing the connected portion. By the supporting member 66, it is possible to prevent the display screen from being adversely affected by a step-difference between the small substrates 61a, and the fraction nondefective from being lowered, thus improving productivity.
However, in the LCD and the manufacturing method thereof as disclosed in Japanese Unexamined Patent publication No. 184849/1996 (Tokukaihei 8-184849) and other publications, due to the LCD structure and the manufacturing process, the following problems (1) and (2) are presented.
(1) The manufacturing process of the described LCD is required to include a step of facing and combining the connected substrate and the large substrate with each other, and the following step of setting the cell gap of the liquid crystal layer which is a spacing between the substrates. Of the two steps, in the step of facing and combining the two substrates with each other, resin to be the seal material and the supporting member (seal material 63 and supporting member 66 in FIG. 19(b)) is formed on a portion of the large substrate corresponding to the outer frame region of the small substrates to be the connected substrate. The main functions of the seal material and the supporting member are (a) to combine the connected substrate and the large substrate with each other and (b) to seal the liquid crystal layer.
Then, the connected substrate is combined with the large substrate in accordance with the pattern of resin. Thereafter, using a pressing device, etc., the substrates are pressed against each other so as to set the cell gap. After setting the cell gap, the resin to be the seal material, which has not been cured yet, is cured so as to form the seal material.
Here, in pressing for setting the cell gap, the resin has not been cured yet. For this reason, the pattern width of the resin is slightly spread by pressing. When the resin is spread in this manner, there is a case that the resin thus spread protrudes into the display screen region of the liquid crystal layer.
In particular, at the connected portion of the small substrates, it is required to narrow the connected portion as much as possible. This is because when the connected portion is wide, the connected portion becomes noticeable on the display screen as the pixel pitch is disturbed at the connected portion, and this makes the display screen to look unnatural as a whole.
The resin protruded into the display screen region in the described manner is shielded by a non-transmissive pattern such as a black matrix (BM). Since the non-transmissive pattern is a non-display region on the display screen, the display screen is prevented from being adversely affected by the protruded resin. However, in order to shield the protruded resin, it is required to widen the region of the non-transmissive pattern.
However, when the non-transmissive pattern is widened only at the connected portion, as described, the connected portion becomes noticeable as the pixel pitch is disturbed only at the connected portion; thus, it is required to widen the non-transmissive pattern with respect to the entire display screen. However, when the region of the non-transmissive pattern is widened, another problem is presented in that the aperture ratio of the pixels is lowered.
The lowering of the aperture ratio of the pixels darkens the entire display screen and the contrast is lowered. As means for preventing lowering of contrast, the backlight may be illuminated with higher illuminance. However, this is not preferable since the power consumption by the backlight is increased, and as a consequence the power consumption of the entire LCD is also increased.
Further, the resolution of the display screen of the LCD whose aperture ratio of pixels has lowered is lower than the resolution of an ordinary LCD having the same display screen size.
Generally, the width of a non-display region between pixels is set in such a manner that the pattern of two rows of the seal material and the connection margin are within the set width. For this reason, a pixel is required to include a non-display region having a certain area. However, in order to improve the resolution, it is required to reduce the area of each pixel in spite of the fact that the area of non-display region remains constant regardless of the pixel area. Thus, in the LCD having the described arrangement, when the resolution is to be improved by reducing the pixel area, the proportion of the non-display region is relatively increased with respect to the pixel area. As a result, the aperture ratio of pixels is further lowered.
Therefore, in the LCD in which a larger screen is realized by connecting a plurality of small substrates to one another, as described, unless the lowering of aperture ratio is prevented, it is difficult to improve the display quality of the LCD. In order to solve this problem, at the connected portion of the small substrates in particular, it is required to prevent resin to be a supporting member from spreading from a predetermined pattern width.
(2) In response to making the screen of LCD larger, the area of the liquid crystal layer sandwitched between the connected substrate and the large substrate is increased accordingly. As a result, it becomes difficult to sufficiently maintain a constant value for the cell gap of the liquid crystal layer, which is the thickness of the liquid crystal layer whose area has been increased, with respect to the entire liquid crystal layer.
When the cell gap is insufficiently maintained, in the liquid crystal layer, there is a case that the liquid crystal is unevenly distributed on the bottom of the display screen by the dead weight. This generates display nonuniformity on the display screen.
The following describes the problem of (1) more specifically. First, as shown in FIG. 20(a), the resin of the seal material 63 or the supporting member 66 on the surface of the counter substrate 62 immediately after screen printing or dispense profiling has a semi-cylindrical shape having a height H1. When the counter substrate 62 is faced and pressed against the plurality of small substrates 61a, as shown in FIG. 29(b), the resin having a semi-circular cross section before pressing process is spread flat by the pressing process.
Note that, in the pressing process, the cell gap is set (also known as "gapping") so that a spacing H2 between the small substrates 61a and the counter substrate 62 constituting the LCD takes a desired value (generally, several .mu.m).
Here, the accuracy of the height H1 of the seal material before pressing process depends on the mechanical accuracy of the screen printing or the dispense profiling, and generally has an error of substantially .+-.10 percent. Accordingly, the width W2 of the seal material after pressing process also has an error of substantially .+-.10 percent. The positioning accuracy of the seal pattern also depends on the mechanical accuracy of the screen printing or the dispense profiling, and therefore an error of several ten .mu.m is generated.
As described, in the seal material as formed by the screen printing or the dispense profiling, the dimension accuracy and the positioning accuracy are both poor. Thus, as described, in the conventional LCD, the problem of (1) is presented, and the problem of (2) is also presented when the screen is made larger. As a result, in the conventional LCD, it has not been possible to realize a large screen LCD while maintaining a high display quality.